In recent years, there has been an increasing interest in networks of small-size, low-power consumption wireless terminals, such as WPAN (Wireless Personal Area Network) or sensor networks. Also, as a system similar to the networks, an active RF tag that transmits wireless signals by itself can be mentioned.
FIG. 46 is a conceptual diagram showing an example of a wireless network of such small-size, low-power consumption communication apparatuses. In FIG. 46, a wireless network 50 includes a communication apparatus AP 10 serving as a parent device and terminal apparatuses STA 1 to STA 5 serving as child devices. It should be noted that, hereinafter, the communication apparatus is also referred to as an AP and each of the terminal apparatuses is also referred to as an STA.
The AP 10 is a communication apparatus which periodically broadcasts control information of the wireless network 50 in a beacon.
Here, the beacon is a control signal which is periodically transmitted from an AP to all STAs included in the wireless network 50. The beacon includes control information, such as transfer rate, which is used for matching communication format used for communication between the AT and the STAs. The STA 1 to the STA 5 are terminal apparatuses which perform data transmission/reception to/from the AP 10, based on the control information.
Wireless terminals used in such the wireless networks are characterized by its low power performance as being small-sized and allowing several years of driving with the use of battery. In such the wireless networks, communication protocols and frame formats that reduce the power consumption of the wireless terminals are used. For example, such the wireless networks each include an active period in which the wireless terminals perform communication and an inactive period in which the wireless terminals do not perform communication and may go to a sleep state in the wireless network. The longer the inactive period is set, the longer the sleep state period can be. This allows reduction of power consumption (for example, see PTL 1).
An example will be shown below in which the power consumption of the communication apparatus is reduced using the inactive period.
FIG. 47 shows a super-frame period 310 which includes an active period 330 and an inactive period 332. The super-frame period is a time period between the AP 10 transmits a beacon 900 and the next beacon 900.
As shown in FIG. 47, the super-frame period 310 has the active period 330 and the inactive period 332. The active period 330 is a time period in which the AP 10 and the STA 1 to the STA 5 are communicable therebetween (the communication functionality is active). The inactive period is a time period in which the AP 10 and the STA 1 to the STA 5 do not perform communication. In the inactive period, the AP 10 and the STA 1 to the STA 5 turn themselves into the sleep state, thereby reducing the power consumption.
When powered on, the AP 10 determines a transfer rate among a plurality of transfer rates supported by the AP 10. Subsequently, the AP 10 and the STA perform packet transmission and reception therebetween at the transfer rate determined by the AP 10.
The active period 330 is shared between the AP 10 and the STA 1 to the STA 5. The AP 10 broadcasts the beacon 900 at the start of the active period 330. The remaining portion of the active period 330 is used for data communication between the AP 10 and the STA 1 to the STA 5.
The beacon 900 includes control information relating to the frame, such as the number of time slots of the active period, allocation of the time slots, the length of the active period 330, the length of the inactive period 332, a time until transmission of the next beacon.
FIG. 48 is a diagram showing an example of a communication sequence between the AP 10 and the STAs.
The AP broadcasts the beacon 900 at the start of the active period (S910). The STA 1 to the STA 5 receive the beacon 900 to acquire the control information. The control information includes information such as the length of the active period 330 and the length of the inactive period 332.
In the active period 330, communication is performed between the AP 10 and the STA 1 to the STA 5. For example, the STA 2 transmits data to the AP 10 (S911), and the AP 10 transmits to the STA 2 an ACK response to the data (S912).
During the inactive period 332, no communication is performed between the AP 10 and the STA 1 to the STA 5. The AP 10 and the STA 1 to the STA 5 can go to the sleep state during a time previously informed of by the control information included in the beacon 900. This allows reduction in power consumption. Then, the AP 10 and the STA 1 to the STA 5 revert to an operational state immediately prior to the end of the inactive period 332 and prepare for the communication of the next active period 330.
After the end of the inactive period 332, the AP 10 starts processing of the next active period 330 and broadcasts the beacon 900 (S913). In the subsequent active period 330, communication is performed between the AP 10 and the STA 1 to the STA 5.
Here, in FIG. 48, the example is shown where communication from the STA 1 is failed. It is assumed that the STA 1 transmits data to the AP 10 and reception error of the data occurs at the AP 10. In this case, the AP 10 does not transmit the ACK response to the data.
The STA 1 waits for the ACK response from the AP 10 for a predetermined time period. Then, if no ACK response returns from the AP 10, the STA 1 determines that the transmission has failed. Thus, the STA 1 attempts retransmission of the packet. Here, it is assumed that the retransmitted packet is successfully received by the AP 10. In this case, the AP 10 transmits to the STA 1 an ACK response to the retransmitted packet and ends the communication.
Subsequently, the AP 10 and the STA 1 to the STA 5 communicate with each other, repeating the same operation.
The IEEE 802.15.4 which is the international standard for the wireless PAN is known as a telecommunications standard for achieving the wireless network 50 of the small-size, low-power consumption communication apparatuses as described above.